1. Field of the Invention
The invention relates to a method of decoding channel symbols included in multi-symbol code words in a data stream which is protected by a symbol error correcting code such as a Reed-Solomon code, including the steps of:
a. sequentially receiving the channel symbols of the code word;
b. demodulating the channel symbol so as to form corresponding code symbols corresponding thereto with indication of the reliability of the demodulation by means of an at least bivalent first indicator flag;
c. collecting the code symbols corresponding to the complete code word in an intermediate memory; and
d. decoding the code symbols to recover the corresponding code word and correcting an unreliably decoded code word by means of redundancy symbols which are included in the code symbols, with indication of the decoding reliability of the corrected code word by means of a second multivalent indicator flag.
2. Description of the Related Art
Such a method is known from the previous Netherlands Patent Application No. 8200207, corresponding to U.S. Pat. No. 4,477,903, issued Oct. 16, 1984, assigned to applicant's assignee. Solomon codes, are also well know. By means of such codes, within a code word of n symbols t incorrect symbols can be located and corrected. Furthermore, e incorrect symbols can be corrected subject to the condition that the locations are known. Finally, d incorrect symbols can be detected. When a code word contains (n-k) redundant symbols, it holds good that (n-k).gtoreq.2t+d+e. Often multiple symbol correcting codes such as Reed-solomon codes are used. Two principles are known in this respect. According to the interleaving principle, successive code symbols of a first code word are distributed, by introducing respective different time delays, between a number of code symbols of a second code word, said number being equal, for example, to the number of code symbols contains in the first code word. In accordance with the product code principle, a group of code symbols is divided into sub-groups in two or more different manners, a code word being formed from each sub-group by the addition of redundant symbols (one or more per sub-group). A substantial number of the code symbols then belong to two (or possibly more) different code words. The channel symbols are derived from the code symbols in accordance with given restrictions in order to adapt the information stream to the transmission capacity of the channel. Examples of such restrictions are:
a lower limit exists as regards the distance between two successive signal transitions in the channel,
an upper limit exists as regards this distance,
the channel signal has no or only a small DC component,
certain channel symbols may be forbidden for reasons other than transmission capacity.
In accordance with the present state of the art, successive channel symbols are separated by three spacing bits. However, in certain applications more spacing bits may be present, or such bits may be absent.
According to the above identified patent, fourteen-bit channel symbols are converted into eight-bit code symbols during demodulation. Furthermore, a flag bit indicates whether the decoding is reliable or unreliable. The latter indication is given when the channel symbol does not comply with one or more of the above restrictions. The spacing bits, which must also satisfy the modulation rules, may or may not be taken into account for the evaluation decoding of the quality. The quality may also be determined on the basis of other properties of the channel signal; for example, the frequency spectrum or the presence of sufficient signal power for a given frequency band, Consequently, the indication given by the flag may have three or more possible values.
Upon decoding, usually first the error-indicating or "syndrome" symbols are formed and on the basis thereof it is decided whether or not a correction is necessary. The correction can usually be performed correctly; however in some cases correction will be impossible because the code word contains too many incorrect code symbols.